Chemotherapy Side Effects – FAQ

Central Toronto Veterinary Referral Clinic

The treatment of cancer among companion animals is becoming increasingly common. Often, a client’s fear of potential side effects will prevent some animals from receiving therapy for very treatable diseases. Following is a list of answers to frequently asked questions to help educate clients about what can be expected with chemotherapy treatments.

Do pets commonly become sick or even die from chemotherapy treatments?

Very few animals become ill while receiving chemotherapy. Approximately 30% of dogs and 10% of cats will experience side effects. Most are self-limiting and manageable at home. Fewer than 10% of patients will require any dose reduction throughout their entire treatment. Having an animal die from chemotherapy is exceeding rare (less than 1%) and is usually from the development of sepsis. Is the experience for pets the same as for people receiving chemotherapy? NO! This fact needs to be made clear. In people the goal of treatment is often cure, while for animals it is usually management to provide an excellent quality of life, for as long as possible. People receive chemotherapy at higher doses, more frequently and with lower WBC counts than do dogs and cats. This level of aggressiveness places people at significantly higher risk for the development of side effects. This is a level of risk not taken in veterinary oncology.

Is the experience for pets the same as for people receiving chemotherapy?

NO! This fact needs to be made clear. In people the goal of treatment is often cure, while for animals it is usually management to provide an excellent quality of life, for as long as possible. People receive chemotherapy at higher doses, more frequently and with lower WBC counts than do dogs and cats. This level of aggressiveness places people at significantly higher risk for the development of side effects. This is a level of risk not taken in veterinary oncology.

What are the most common side effects of chemotherapy?

Chemotherapy impacts rapidly dividing cells. The susceptible cell populations are cancer cells, cells of the GI tract, the bone marrow, hair follicles and reproductive cells. In treatment, we are happy to be rid of the cancer cells, as that is the ultimate goal of therapy. GI side effects are the most commonly noted chemotherapy side effects. We can see vomiting, diarrhea, loss of appetite or some combination develop. Bone suppression can result in a transient decrease in WBC numbers. Usually the neutrophil count will not drop low enough to be of any clinical concern. In 5-8% of cases, moderate to severe neutropenia may develop which will require intervention and treatment and will result in a need to lower the dose of certain chemotherapy drugs moving forward with future treatments. Hair loss in cats is rare, though the loss of whiskers is commonly associated with doxorubicin treatment. Hair loss in dogs is most commonly seen in dogs with continuously growing coats. Most dogs will not lose all of their hair, many will lose their course overcoat and develop a fine “puppy coat”. Smaller dogs and those with lighter coloured hair coats appear to be at highest risk for hair loss. Cat’s whiskers and dog’s hair will grow back after chemotherapy has finished. Usually, because of prior spaying or neutering, reproductive cells are not an issue, though intact animals should not be bred.

When side effects happen, when do they start and how long do they last?

The onset of GI side effects is usually seen 3 to 5 days after the chemotherapy treatment and in most cases the signs are self-limiting and resolve in 12-24 hours. I tell owners to worry if they see more than 2 episodes of vomiting or diarrhea or more than one day loss of appetite. Bone marrow suppression usually happens 7 to 10 days after treatment. A low WBC count may be an incidental finding or may be associated with illness, though clinically significant leukopenia is uncommon. What happens if side effects occur? Owners should contact the attending veterinarian. I always take the temperature (remembering animals with very low WBC counts will be unable to mount a fever) and a CBC. Vomiting and diarrhea can be managed by dietary changes, anti-nausea or anti-diarrhea medications. Though rarely seen, animals with very low WBC counts or severe dehydration from vomiting and diarrhea may require hospitalization for IV fluids and IV antibiotics. The average hospitalization is 2 days. Chemotherapy can usually continue, with some dosing adjustments.

What happens if side effects occur?

Owners should contact the attending veterinarian. I always take the temperature (remembering animals with very low WBC counts will be unable to mount a fever) and a CBC. Vomiting and diarrhea can be managed by dietary changes, anti-nausea or anti-diarrhea medications. Though rarely seen, animals with very low WBC counts or severe dehydration from vomiting and diarrhea may require hospitalization for IV fluids and IV antibiotics. The average hospitalization is 2 days. Chemotherapy can usually continue, with some dosing adjustments.

Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.

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